The invention is related to continuous digestion of fiber material and especially impregnation of the fiber material during transport to a digester. Such pre-impregnation of fiber material comprises a very important part of the delignification process and has as an object to replace the air or gas content of the fiber material with an impregnation liquid or with cooking liquor.
The driving off of air or gas in fiber material so that it may be impregnated is commonly done by means of steaming at a relatively small superatmospheric pressure with succeeding submerging into cooking liquor at higher pressure and temperature but also by means of other methods, e.g., first vacuum treatment or pressing and then submerging into a liquid before the cooking starts. Whether the impregnation is performed in one way or the other, certain equipment is needed which both, from an economical as well as from a space point of view, is to be limited as much as possible during simultaneous consideration that the impregnation should be as effective as possible. Fiber material, such as it arrives to a cooking installation, is made up of a heterogenous mixture of comminuted raw material, e.g., wood, grass, reeds, straw, etc. Even if coarse screening has been done, the variations from piece to piece are considerable both in size, shape and density. Especially with mixed raw material and with the popular use of "whole trees" as the raw material, the differences and the need for selective impregnation are accentuated.
According to the present invention, an effective impregnation is effected (possibly after steaming) by means of varying impregnation time in a simple and effective manner while the fiber material is surrounded by liquid. In this way, the heterogenous character of the fiber material is compensated for, and the pulp yield, evenness and strength characteristics are improved. Impregnation with time variations for separate fiber material particles is in itself previously known, e.g, through Swedish Pat. No. 174,656. The mentioned patent has as its main characterizing part, in an impregnation space separate from the digester itself, replacement of the air in the fiber material cells by cooking liquid by means of pressure variations and by provision of a selective impregnation of the fiber material in such a manner that the fast impregnatable fiber material is removed earlier from the impregnation space than the more difficult impregnatable fiber material. The impregnation process takes place in a standing impregnation vessel arranged at an angle to the horizontal plane, to which a mixture of fiber material and cooking liquor under pressure is fed by means of a first device and fed out by means of another device. The vessel is equipped on the inside with a screw conveyor for fiber material which has sunk in the cooking liquor and, at the same time, air is being expelled through a valve and the pressure variations are obtained by a vacuum pump and/or by tapping off liquid respectively by pumping in of cooking liquor and/or by use of pressure accumulators.
Improved impregnation can be effected according to the present invention in a simple and economically advantageous way be in connection with a Kamyr conventional continuous digester, the transition of fiber material from low to high pressure being accomplished by a conventional Kamyr high-pressure feeder. The fiber material is transported in liquid which is pumped to the digester top where the liquid is separated and returned back to the high-pressure feeder for renewed use as transport medium. Such a feed system is per se known, as illustrated by U.S. Pat. No. 3,802,956. In U.S. Pat. No. 3,802,956, from a high-pressure feeder, fiber material and liquid are pumped through a line up to the top of an impregnation vessel (separate from the digester) where liquid is separated off by a screen assembly and led back to the pump through a return line. The high-pressure feeder is conventional and primarily consists of a rotor equipped with through-going pockets in a housing equipped with inlet and outlet connections. When a rotor pocket is in vertical position, a mixture of liquid and fiber material is fed into the feeder, and in order to make the filling more effective, liquid is extracted at the lower end of the feeder housing through a screen to a pump which circulates liquid back to the feed-in line. Before the high-pressure feeder, the fiber material and liquid are normally at a small superatmospheric pressure of about 1 atm, while in transfer valve, high-pressure line fiber material and liquid can be exposed to a pressure corresponding to the cooking pressure, e.g., 10 atm. The pockets and the housing of the high-pressure feeder are designed so that one of the pockets is always being filled at the same time as another pocket is being emptied, whereby the flow of fiber material in the transfer line is continuous. Before the fiber material arrives at the high-pressure feeder, it has usually been treated with steam, whereby the greater part of the air has been driven out of the pores of the fiber material. The real impregnation with cooking liquor takes place during entrance into the cooking liquor which circulates through the high-pressure feeder and through the feeding line to the top part of the digester at full cooking pressure, whereby the greater part of the fiber material quantity, e.g., by means of a screen device, is separated from the transport liquid which, in the above-mentioned manner, is led back to the circulation pump. If the separation of transport liquid and fiber material in the digester top is done in a conventional manner with a so-called top screw surrounded by a concentric screen plate, mainly the smallest fiber particles which can pass the screen openings will follow the transport liquid back to the circulation pump, feeding apparatus -- feeder, and back to the digester top again until the particles fasten on any larger fiber particle and continue downwards in the digester. This extra circulation which means an extra impregnation time is generally undesirable since the small particles which can pass the screen in the digester top already are thoroughly impregnated and, therefore, do not need an extra impregnation time. Instead, it is desirable that larger fiber particles which are not sufficiently impregnated should receive an extra retention time before they are digested. This has been accomplished according to the invention in a surprisingly simple and effective manner.
According to the present invention, the fiber material that should have a longer retention time in impregnating liquid is retained longer, while the fiber material that does not need a longer retention time is not, by the substitution of a stilling well for the inlet-outlet arrangement at the top of the digester instead of the screw-feeder with screen as is conventional (see U.S. Pat. No. 3,802,956), and by directly connecting the high-pressure transfer valve to the stilling well. This allows the elimination of the impregnation vessel in U.S. Pat. No. 3,802,956 and in copending application Ser. No. 719,656, without subsequent loss of its function -- that is, proper impregnation of all of the fiber material is still achieved. While the utilization of a stilling well per se is known (see German Offenlegungsschrift No. 2,361,627) previously in the utilization of such a stilling well the practice has been to separate out all of the particles from the liquid while not utilizing screens and, therefore, such stilling wells have only been utilized in areas where the fiber material is always more dense than the liquid, or accessory means were employed to facilitate removal of the fiber material (see copending application Ser. No. 659,638). The method and apparatus according to the present invention are distinct from the prior art in that by design, a selected portion of the fiber material is recirculated with the liquid withdrawn from the stilling well and that selected fiber material is subjected to impregnation conditions for sufficient additional time so that eventually impregnation does take place before digestion.
In mill operation, the practice has been to install steaming/impregnation apparatus designed so that a large part of the raw material will be completely or in any case sufficiently impregnated, while a smaller part will remain "hungry" for chemicals, and this latter raw material part will, after treatment, remain partly uncooked, i.e., higher reject, especially at higher yields. It is typical that, depending upon the degree of impregnation, a portion of fiber material (e.g., chips) tends to either sink or to float. A chip can sink in water but still float in cooking liquor with higher specific weight. A chip piece which sinks in cooking liquor can generally be designated as sufficiently impregnated.
The present invention takes advantage of this physical phenomenon in order to reduce the total dimensions of the equipment through selective separation of "sinkers" and "floaters", especially in an established feed system for a continuous digester, in that the "sinkers" sink down in the digester cooking zone while the "floaters", which normally constitute a relatively small part of the total quantity, pass through the impregnation process a second time.
It is the primary object of the present invention to provide a simplified but effective method and apparatus for complete impregnation of all fiber material from a heterogenous source before digestion of that fiber material. This and other objects of the invention will become apparent from the detailed description of the invention and from the appended claims.